Continuous and semi-continuous processes for creating patterns on extruded plastic sheets have been used in the building components industry for a number of years. Some prior systems have disclosed rigid linked patterns for forming shaped impressions in an extruded sheet material. Unfortunately, such rigid shaped patterns tend to form unsightly horizontal seams in the material. Other systems have used pattern forms on rotating cylindrical drums. Although these processes are continuous, and do not produce horizontal seams, they often require expensive additional equipment and instrumentation to align the arcuate surface of the pattern with the relatively flat surface of the product, and to avoid, or correct, unwanted bowing of the product.
Because of the limitations on prior continuous processes, some manufacturers have opted for injection or blow molding such products one at a time. While this technique can provide the desired detail in texture and surface finish, it is limited to product sizes of about 4-5 feet in length and provide product thicknesses which are practically limited to greater than about 0.080 inches. This is generally because of the difficulty associated with flowing hot viscous polymer through thin cross-sectional profiles in steel molds. Additionally, because of the known size limitations, the randomness of individual features on the surface of a molded product is limited. This is because only a relatively small number of pattern elements, such as shingles, can be molded into the relatively small surface area. When several of these products are aligned side by side on a wall of a building, for example, it is sometimes obvious to see the pattern repeated over and over again.
One solution to these problems is disclosed in Bosler, U.S. Pat. No. 5,314,325 dated May 24, 1994. Bosler's invention provides a continuous vacuum forming process which uses a resilient mold belt for providing semi-continuous production of almost unlimited lengths of shaped polymeric articles. He further teaches a single cooling step whereby the entire width of the product is brought below the heat deflection temperature of the polymeric material.
It has now been determined, however, that a single cooling step reduces the temperature of the edge portions below a comfortable range for subsequent sizing and forming. Accordingly, there remains a need for providing for the separate formation of edge portions, such as butt-edges and nail edges customarily used in the fabrication of siding panels.